A graphite material has a high heat resistance under a non-oxidizing atmosphere and has been widely used for various uses which requires heating to high temperatures, such as a graphite heater, a jig, and a component for a device.
However, a graphite material has been known to have characteristics varying according to the exposure temperature. Particularly, for the specific resistance, as distinct from a metal material, or the like, the specific resistance decreases with an increase in temperature, and when the temperature exceeds a given temperature, the specific resistance turns around to increase.
Methods using the graphite material as a heat source include a method involving arc discharge at the tip part and a method utilizing Joule heat generation from the main body. In either method, the specific resistance at high temperatures is important. In the method involving arc discharge, a lower main body resistance results in more efficient power supply, and hence there is a demand for a lower specific resistance product. In contrast, in the method utilizing Joule heat generation, a high specific resistance product tends to be demanded for efficient heat generation. When the resistance at room temperature is high, the resistance at high temperatures tends to be greatly reduced. Accordingly, a balance between the resistance at room temperature and the resistance at high temperatures is important considering the restriction of the maximum voltage or the maximum current of the power supply device. Further, for a material with a small reduction rate of the specific resistance at high temperatures, the resistance at room temperature tends to be lower. Thus, it is necessary to reduce the design cross section of the heater in order to obtain the necessary heat generation amount, and the durability therefore tends to be inferior.
In regard to such a problem, PTL 1 discloses a method for manufacturing an isotropic graphite having a high specific resistance by adding a graphite powder to a coke powder as a raw material filler. However, PTL 1 does not mention the behavior of the specific resistance at high temperatures, and no sufficient study has been made therein.
PTL 2 proposes a graphite material obtained by adding a titanium element, an aluminum element, and a boron element, in addition to a coke powder and a graphite powder, as a raw material filler for suppressing the resistance change rate at high temperatures within 15%, and a manufacturing method thereof. The metal type elements included in the graphite material described become impurities and hence are not preferable in a semiconductor manufacturing device or the like.
Further, PTL 3 proposes a graphite material with the specific resistance at 1600° C. kept high. However, reduction by around about 30% is observed as compared with the value at room temperature, and the output region of the power supply device is required to be designed wide. Thus, the graphite material is still not preferable.